Impact ionization has been known for several years. U.S. Pat. No. 4,432,075 to B. Eitan and U.S. Pat. No. 4,821,236 to Hayashi et al. describe an EEPROM transistor adjacent to a charge generator, creating a substrate current near the EEPROM, creating excess charge or holes, resembling space charge, near subsurface electrodes of the EEPROM. Assume that the holes are generated and accelerated toward one of the electrodes of the EEPROM. Resulting secondary electrons are sufficiently energetic to penetrate gate oxide over the substrate and become injected into a conductive floating gate. For very small EEPROMs, the floating gate becomes charged by band-to-band tunneling, a situation which eliminates the need for a control gate over the floating gate.
U.S. Pat. No. 6,690,059 to B. Lojek and assigned to the assignee of the present invention discloses a nanocrystal EEPROM transistor where charge is stored in a nanocrystal layer. Charge is supplied from a charge storage layer for facilitating electron communication between the two layers. Other EEPROM nanocrystal devices exist in the prior art without the charge storage layer, such as U.S. Pat. No. 6,320,784 to Muralidhar. In the '784 patent, charge is moved or removed from nanocrystal elements using traditional electrodes existing in non-nanocrystal EEPROMs.
One of the problems that occurs is that either high voltages or relatively large currents are needed for programming many nanocrystal EEPROMs. Where programming is by Fowler-Nordheim tunneling, large voltages are needed. To obtain high charge, charge pumps are frequently employed. This means additional circuitry, including switches, capacitors and an oscillator. Where programming is by hot electrons, a good current is required. This, too, requires additional circuitry. An object of the invention was to devise a nanocrystal EEPROM that could be programmed at low voltage and with low current.